The Space Pen (also known as the Zero Gravity Pen), marketed by Fisher Space Pen Company, is a pen that uses pressurized ink cartridges and is able to write in zero gravity, underwater, over wet and greasy paper, at any angle, and in a very wide range of temperatures.

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Austrian Friedrich Schächter, creator of the overall concept for a "Space Pen" ("Weltraumkugelschreiber"),

American industrialist and pen manufacturer Paul C. Fisher, who invented the "thixotrop special ink",

Austrian mechanic Erwin Rath,

and is manufactured in Boulder City, Nevada, United States of America. Paul C. Fisher first patented the AG7 "anti gravity" pen in 1965. Pens claiming some or all of the same abilities have also appeared on the market from other manufacturers.

There are two prominent styles of the pen: the AG7 "Astronaut pen", a long thin retractable pen shaped like a common ballpoint, and the "Bullet pen" which is non-retractable, shorter than standard ballpoints when capped, but full size when the cap is posted on the rear for writing.

Several of the Fisher Space Pen models (the "Millennium" is one) are claimed to write for a lifetime of "average" use; however the product literature states that the pen will write exactly 30.7 miles (approximately 49.4 kilometers).

Standard Space Pen refills can be used in any pen able to take a standard Parker Pen Companyballpoint refill, using the small plastic adapter that is supplied with each refill. Fisher also makes a Space Pen-type refill that fits Cross pens, one that fits 1950s-style Papermate pens (or any pen that uses that type of refill), and a "universal" refill that fits some other ballpoint pens.

The ballpoint is made from tungsten carbide and is precisely fitted in order to avoid leaks. A sliding float separates the ink from the pressurized gas. The thixotropic ink in the hermetically sealed and pressurized reservoir is able to write for three times longer than a standard ballpoint pen. The pen can write at altitudes up to 12,500 feet (3800 m). The ink is forced out by compressed nitrogen at a pressure of nearly 35 psi (240 kPa). Operating temperatures range from −30 to 250 °F (−35 to 120 °C). The pen has an estimated shelf life of 100 years.

One of the first patents on space pen is US3285228, which was filed on 19 May 1965.[1]

An urban legend states that NASA spent a large amount of money to develop a pen that would write in space (the result purportedly being the Fisher Space Pen), while the Soviets just used pencils.[2][3] There is a grain of truth: NASA began to develop a space pen, but when development costs skyrocketed the project was abandoned and astronauts went back to using pencils, along with the Soviets.[2][3] However, the claim that NASA spent millions on the Space Pen is incorrect, as the Fisher pen was developed using private capital, not government funding. NASA – and the Soviets[3][4][5] – eventually began purchasing such pens.

NASA programs previously used pencils[6] (for example a 1965 order of mechanical pencils[7]) but because of the substantial dangers that broken pencil tips and graphite dust pose to electronics in zero gravity, the flammable nature of wood present in pencils,[7] and the inadequate quality documentation produced by non-permanent or smeared recordkeeping, a better solution was needed. Russian cosmonauts used pencils, and grease pencils on plastic slates until also adopting a space pen in 1969 with a purchase of 100 units for use on all future missions.[8] NASA never approached Paul Fisher to develop a pen, nor did Fisher receive any government funding for the pen's development.[7] Fisher invented it independently and then, in 1965, asked NASA to try it. After extensive testing, NASA decided to use the pens in future Apollo missions.[6][8][9] Subsequently, in 1967 it was reported that NASA purchased approximately 400 pens for $6 a piece.[8]